Physical parameters of Ga1-xAlxN and Zn1-xMgxO wurtzite crystals (x = 0 and 0.5) were analyzed using first-principles calculation. A plane-wave pseudopotential method was employed to a density functional theory, and the effect of alloying on the lattice parameters, spontaneous polarization in the c-axis direction, elastic stiffness and piezoelectric stress constants were calculated. Polarization effect in the heterosystems of Zn-polar Zn0.5Mg0.5O/ZnO and O-polar ZnO/Zn0.5Mg0.5O as well as Ga-polar Ga0.5Al0.5N/GaN and N-polar GaN/Ga0.5Al0.5N was compared by estimating their interface charges. The interface charges of the former two systems were estimated to be ∼5 times larger than those of the latter two assuming the strained structures by pseudomorphic growth.
We have investigated the narrowband ultraviolet (UV) emission characteristics of Al0.94Gd0.06N grown by reactive magnetron sputtering in an ultra-pure process. A resolution limited, narrow band luminescence line from Gd3+ ions has been observed at ∼320nm. The crystallographic properties of the Al0.94Gd0.06N thin films are sensitive to the growth temperature. With a decrease in the growth temperature, the emission efficiency has been found to increase considerably. According to the crystallographic properties investigated by extended X-ray absorption fine structure analysis, fluctuation of the column-III sublattices around the Gd3+ ions plays a key role to enhance the luminescence intensity.
We investigated the growth temperature dependence of luminescence properties in Eu-doped GaN layers grown by organometallic vapor phase epitaxy (OMVPE). The dominant photoluminescence (PL) peak intensity at 621 nm, due to the intra-4f shell transitions of 5D0-7F2 in Eu3+ ions, became the highest when the sample was grown at 1000°C. Above 1000°C, the PL peak intensity decreased because of the lower Eu concentration associated with the surface desorption of Eu ions. On the other hand, although the Eu concentration of the layer grown at 900°C was only half of the layer grown at 1000°C, the pronounced decline in the PL peak intensity was observed with decreasing growth temperature from 1000°C to 900°C, which results from the modification of the local structure around Eu ions. These results indicate that the growth temperature strongly influences the Eu concentration and the local structure around Eu ions. Therefore, an optimized growth temperature exists for strong Eu-related luminescence from Eu-doped GaN layer grown by OMVPE.
As a safe, simple, and low energy-consumption growth method of oxide semiconductors, we have developed a mist-chemical vapor deposition (CVD) technique. This technique has been applied to homoepitaxial growth of zinc oxide (ZnO) semiconductor thin films on ZnO bulk substrates. Step-flow growth has been achieved at the growth temperatures of 950∼1000°C. The step height was 0.26nm, that is, one molecular-layer height, in the initial growth stage (20∼50nm in ZnO epilayer thickness) and it is enhanced as 0.52nm, that is, two molecular-layers height, for the thicker (>200nm) films, due to coalescence of two terrace layers. Terrace shapes have been well interpreted in terms of the balance between incorporation and evaporation of adatoms at the terrace edges. The homoepitaxial ZnO layers exhibited good crystallinity comparable to that of substrates, but the inclusion of impurities, probably from the source materials due to their insufficient purity as semiconductor sources, remained as one of the most important problems to be overcome for future evolution of this unique growth technique.
ZnO thin films were prepared from an aqueous zinc nitrate solution by an electrochemical deposition method. To control the growth rate of the ZnO films, electrochemical deposition was carried out by applying constant current density, rather than constant potential as in conventional methods. The preferred orientation of ZnO films deposited on polycrystalline Au electrodes changed depending on the cathodic current density. The grain size and grain shape could also be controlled by changing the current density. An epitaxial layer of Pt(111) on c-sapphire was also used as an electrode. ZnO films prepared by application of high current densities exhibited not only (0002) orientation but also other orientations. At low current densities, the deposition formed not a film but rather isolated hexagonal columnar grains on the electrode. At intermediate cathodic current densities, epitaxial ZnO thin films were obtained. At 100mA/cm2, a continuous epitaxial ZnO thin film without pits or spaces between grains was obtained. Therefore, it is expected that the proposed electrochemical deposition method has potential to be applied to preparation of functional materials with well-controlled structures.
We propose a novel corundum-structured oxide semiconductor alloy system of α-(Ga2O3)-(Fe2O3)-(Cr2O3) thin films as a potential material with multifunctional properties contributing to future unique devices. In this paper the focus is given on the detailed characterization of α-Fe2O3 thin films grown on c-plane sapphire substrates. α-Fe2O3 is an oxide semiconductor with the optical band gap of 2.2eV, exhibiting a weak ferromagnetic property, and is alloyed with α-Ga2O3 to develop promising potentials toward spintronic applications. The growth has been carried out by the mist chemical vapor deposition (CVD) method which has been developed by our group. In the experiment, we used water solution of iron acetylacetonate [(C5H8O2)3Fe] as a Fe source and air as carrier gas. The growth temperature was set at 500°C. X-ray diffraction measurements revealed the successful formation of corundum-structured α-Fe2O3 thin films. The full-width at half maximum of X-ray diffraction rocking curve was as small as 41arcsec. In-plane pole figure measurements indicated that α-Fe2O3 thin films were epitaxially grown on the substrate, but that rotational domains were contained with the volume ratio of about 0.7%. The atomic force microscope surface image showed many grains and the root mean square roughness was 1.26nm. These results showed the successful fabrication of highly-crystalline epitaxial α-Fe2O3 thin films on c-plane sapphire substrates, though further efforts to improve the surface morphology were suggested.
Photoluminescence (PL) properties of Eu-doped ZnO (ZnO:Eu) grown by a sputtering-assisted metalorganic chemical vapor deposition technique were investigated. In PL measurements at 300K, the samples annealed at 600°C for 30min showed clear red-emission lines due to the intra-4f shell transition of 5D0→7FJ (J = 2, 3) in Eu3+. In photoluminescence excitation (PLE) spectra, the PL were observed under the high-energy excitation above the band-gap energy of ZnO (indirect excitation) and the low-energy excitation resonant to the energy levels of 7F0-5D3 and 7F0-5D2 transitions in Eu3+ (direct excitation). These results revealed that the energy transfer from ZnO host to Eu3+ was accompanied under indirect excitation in ZnO:Eu.
Three dimensional (3D) recording using two-photon absorption (TPA) is one of the most promising candidate for achieving larger capacity on optical memory. We demonstrated application of organoboron polymer introduced different substitution group to 3D optical memory with waveguide structure. The recording power of thiophene type organoboron polymer was lower than that of phenylene type material. Moreover, three dimensional recording and new reading out method which used CCD camera were tried. As a result, the recorded bits were scattered by the refractive index change and observed by the CCD camera as a clear two dimensional image, from which the information of multiple bits was obtained simultaneously. These results promise that the novel organoboron polymers newly synthesized and the new reading method of the recorded bits organoboron polymers are suitable for 3D optical memories.
The recent trend in electronics device towards bonding large chips to PCB boards by means of solder joint and intermingling with many others assistant materials, such as selectable underfills. This means a larger differential thermal expansion and deformation mismatch between the board, the chip and others assistant materials. To reduce the thermal stresses and strains at solder joints, a lot of underfills have been developed and customarily added to fill the deformation between the chip and the PCB board. In this research, firstly, we show presence of local mismatch of thermal deformation between solder and alloy connector. Secondly, we discuss the effects of the underfill on the reduction of solder joint stresses and strains with deformation mismatch of thermal expansion based on a finite element analysis. This procedure has typically at least moderated the thermal stress at solder joint by a factor of 3, as compared to lower CTE (Coeff. of Thermal Expansion) of underfill. Finally, we probe a method to minimize the deformation mismatch of solder joint by means of optimizing properties (Young's Modulus, Poisson's Ratio and CTE) of the underfill to reduce stresses and strains at solder joints. The optimum method presented here can effectively improve the reliability of solder joint of electronics package by optimizing the properties of underfills, and the method is useful to select or develop underfill materials.
In flexible printed circuits (FPCs), polyimide (PI) resin, which has low dielectric constant, is used for the substrate, and copper, which has low resistance, is used for the wiring. In general, a surface modification technique is applied to the PI resins because the adhesion between PI and copper is poor. Oxygen plasma treatment is a modification technique that should improve adhesion. The treatment increases fine surface roughness and activates the PI surface by introducing functional groups. In this study, the effects of plasma treatment on the surface properties of PI film and on the adhesion of copper plating film to PI film were investigated for high-quality FPCs. In the RF-reactive ion etching (RF-RIE) plasma treatment, oxygen was used as the processing gas. It was at a flow rate of 30 sccm for 3∼20 minutes. The adhesive strength of the copper plating film to PI film was measured using a 90° peel tester. The oxygen plasma treatment improved the adhesion, and the adhesive strength of around 400 N/m is close to a practical level.
The failure loads of FRP bolted joints depend on the failure mechanisms reflected in the failure modes. The aim of this paper is to obtain the failure properties of bolted joints in pultruded CF/GFRP laminates based on the experimental results. The failure modes are divided into bearing, shear-out, tension, shear-tension and bearing-tension failures and the latter three modes are with the net-tension failure of outer layers. Bearing failure could be ductile due to continuity of the compressive failure decreasing the stress intensity and depend on both compressive failure progress and out-of-plane shear failure. Shear-out failure could be ductile due to both decrease in the shear stress intensity for outer layers and stress redistribution among layers during failure progress, and its load could increase with the ratio of width or edge distance to bolt diameter when the loading stress at failure is out of relation to them. Bearing and shear-out failure loads could depend on the volume fraction of outer layers, that is, they could decrease with decreasing volume of outer layers for compressive failure less concentrated on loading point and relatively linearly with for more concentrated. Tension failure load could depend on edge-side interlayer delamination, suggesting that it could appreciably decrease for the small volume of outer layers due to increasing compressive stress and not largely depend on that volume for the thorough delamination of unidirectional layer due to the extremely higher stress at loading point than around by non-restrictive interlayer shear deformation. Shear-tension failure load could increase with the volume fraction of outer layers and bearing-tension for the thorough delamination of unidirectional layer almost linearly with, due to the compressive stress depending on the interlayer property.
Core specimens are drilled from the concrete structures repaired by silane-type impregnant and flexible polymer modified cementitious coating materials (PCM) after long term exposure to investigate the performance of those repair materials. Physical properties of the materials are measured and instrumental analysis are conducted to observe the condition of cement particles hydration and polymer film formation. Silane-type impregnant still maintains the performance after 20years at the impregnated depth of 2∼10mm, also flexible PCM is more waterproof and water vapor permeable than initial. Adhesive strength of flexible PCM increased to twice the initial value after 5years, and maintains the performance after 20years. Elongation of the flexible PCM increased after 20years although it decreased after 2years. Hydration of cement didn't almost proceed and the polymer film had almost no damages after 20years.